The structure in accordance with the present invention is in many respects a further development of earlier foundation structures developed by the applicants. Both the earlier constructions and the present invention relate to structures comprising a number of substantially vertically disposed columns rigidly interconnected with cross-members and forming support for a working platform.
Applicants' earlier platform structures and other known designs have been assumed adequate for sea depths up to 200 meters. With larger depths these earlier designs are however neither technically nor economically assumed adequate. With depths exceeding 200 meters one has hitherto used assumed floating structures as the only or the best solution, and platform structures of the so-called semi-submersible type are most usual, and such structures are therefore also used in a large degree in connection with drilling and production of offshore gas and oil sources.
The main reason for that submersible foundations and platforms have been considered without interest for depths from 300 to 400 meters, is obviously that such structure necessarily will attain almost prohibitive dimensions. Fact is, however, that the size itself implies today not a design obstacle. The structures, particularly made in concrete, must however necessarily be made ashore or along the shore, and thereafter be barged or floated out to the predetermined location and be sunk down in correct upright position. Extensive investigations have shown that the building as well as the barging of such large structures will imply large difficulties. Firstly, the structure as such will demand larger great sea depths at the building site and furthermore the depth during the travel from the building site to the final offshore location must necessarily have the same barging depth. When the structure shall be for instance 300 to 400 meters high, it may be necessary to barge the structure in horizontal or tilted over position. The structure being erected to vertical position on the site of location. In this case, number of problems will arise in connection with the stability, buoyancy and furthermore in connection with the erection and submersion and positioning of the structure on the offshore site. it has been proposed to make such foundation structures in sections with assembling on the site of location, but hitherto no dependable solutions have been presented. Furthermore, one has evaluated various forms of auxiliary means such as barges, floats, buoyancy tanks, in order to provide the necessary buoyancy and stability during the barging and submerging operation. Technical solutions to solve the problems in this way are fully possible, but in addition to be rather complicated they fall much too expensive in order to be economically attractive.
Another essential problem with foundations for such depths concerns the stability when a such high structure is exposed and subjected to the impacts from winds, streams, waves, erosions, etc.
A special problem is to obtain sufficient stiffness and strength within the concept of using an open frame structure design.
A main object of the present invention is therefore to provide an improved design for an offshore foundation structure made in concrete and attractive for depths from for instance 300 to 500 meters and which design can be attractive from a point of view of economy, stability and simplicity in design.
A further object of the invention is to provide a design which makes it possible to keep the width of the structure and also the total weight of same within reasonable values and still to obtain the required strength and stability on site without requiring further stabilization means such as guylines substratum foundations etc.
A further object of the invention is to provide a design which renders it possible to adapt the design to various sea depths for instance in the range of 300 to 500 meters.
A special object of the invention is to provide a design which can make it comparatively easy to barge the structure out in horizontal position to the site of location and carry out the erection and submersion of the structure with minimum requirements to auxiliary buoyancy means etc.
The foundation structure in accordance with the invention is designed to rest firmly on the sea floor in offshore position and it comprises a rigid open frame structure of concrete including at least three upright supporting columns, the lower portions of which are rigidly interconnected with cross members and which uppermost are supporting a working platform in position above sea level, and the structure in accordance with the invention is characterized in that it comprises a lower set of columns arranged with an incline and an upper set of columns extending vertically up to the working platform, the lower set of columns and the upper set of columns being interconnected by means of a rigid box structure or intertie, said box structure or intertie constituting a ridgifying interlink between the lower and upper set of columns and furthermore providing added buoyancy and stability to the foundation structure.
In a preferred embodiment the lower and upper set of circular columns are three in number and arranged as an equilateral triangle when viewed in plan view, i.e. a tripod structure.
In a further embodiment the intertie is composed of self-contained box girders having a through-going opening in the centre zone. In a further preferred embodiment, from the working platform all the way down to the bottom of the structure extends a vertical column primarily serving as a downlead or conductor for drilling equipment, oil and gas piping etc.
In a further preferred embodiment also the cross members connecting the columns at the bottom of the lower set of columns are formed as a box structure similar to the intertie between the lower and upper set of columns. In this preferred embodiment the lower columns are furthermore in per se known fashion provided with frusto-conical enlarged footings, giving the structure an increase in the effective biasing width and thus the stability.
The structure in accordance with the invention possesses a number of advantages and meets most of the aims and requirements forming the object of the invention. A special advantage is that the lower part of the foundation structure including the intertie structure can be made as a standard or module unit having more or less fixed dimensions, while the number, dimension and particularly the height of the upper set of vertical columns can be varied to adapt the structure varying sea depths within a certain depth range.
A special advantage with the foundation structure in accordance with the invention is that it will have improved stability in semi-submerged, floating position due to the buoyancy of the intertie, a quality which makes it possible to locate more loads on the platform such as pumps, oil production equipment etc. When the foundation structure shall be floated to site in upright position this is a large advantage since it reduces the necessity for mounting such equipment on site. Experience has shown that on site mounting falls very costly and difficult.